Is the processor's cooling fan sometimes not coming on after it sits for a few days? That alone would cause it to lock up or not start as it does not take the CPU long to heat up. Just an idea. Also, sadly, the connection pins on cables and what not like hard drives are not often pure gold plated so they can have corrosion even from the factory. Perhaps a q-tip and a bit of DeOxit on some of the internal connections would help.

I used to have a PC system that exhibited similar symptoms in cold weather. I had thought it was a sagging 5V standby power line (+5VSB) that remains powered even when the main local mobo power is off, but it gave the same trouble with a different power supply. It turned out to be the PS_ON# line from the mobo back to the PSU that was being held in the off state (high) until it was warm enough. I fitted a switch that I could use to hold this line low so I could leave the PC with power on for 10 minutes from cold until the Mobo got warm, then I could open the switch and boot the operating system normally. I never got round to fault-finding the original problem, and the PC eventually got replaced by a more modern system.

You could try a different power supply, and if that makes no difference, look at the PS_ON# line to see that it gets brought low. You can check this by attaching a low-power LED (with current limit resistor) between the +5VSB line and the PS_ON# line on the PSU lead to the mobo. If the LED comes on when you hit the ON button and stays on, these two lines are probably OK, and then it's a matter of going round checking the various power connectors and also the processor socket, as Jack suggested.

If your system can boot but freeze after or during booting, it may be a mobo and not a power supply problem. If this is a new system, is it still under warranty?

"I know, I could cure that with pluggin' it into a permanent Mains socket,"

How do you know this? Most of the issues I have when the OS balks on start up can be contributed to abrupt shut-offs, registry cleaning issues and maybe one of the issues you are having, that I have yet to identify :)

But, good luck and if you find the answer, a hard answer, this would be most welcome.

Hello Boswell...
I actually had a similar problem with 2 former PCs on the same spot with different components, except they all had Asus MoBos and Intel CPUs.
That WAS a temperatur problem, since the machine room gets rather cool during winter time... but when this machine showed its symtoms it was around
16 degrees Celcius and warmer. That should not cause any problems... I also thought of moisture, but with around 50 % it is dry enough for anything...
I will check this PS-on line and see what it does. This sounds very much like it could be ( a part of? ) the problem...Maybe I can adjust any setting to change the switch threshold.

Hello Jack, hallo Space...
The whole thing starts up with all peeps and fans and drives....it just stops at some point. One time, I even went into Nuendo and loaded a project before it froze...
After 10 minutes I re-tried and it worked 17 hours w/o any crash or problem. Also, for the rest of the week there were no boot-up issues, nor any other hick-ups.
The PC and all components are brand new, all plugs proberly seated, RAM modules also seated tight and stress-checked , registry is cleaned and all 4 drives are defraggled as good as it gets...
The first thing I checked was for virus or other failures in OS and system, but to no avail. The fault appears only if the PC was disconnected from mains, as described above, for longer then 2 days.

Well, I will find out what it causes, eventually....
Maybe if I just give it a size 13 kick??..

lol

Thank you all for your input...

Big K, post: 355835 wrote: ...The PC and all components are brand new...

Take it back to the store.

From your further description, it sounds like a motherboard boot problem and not a problem with the PS_ON# line or power supply.

Agreed. This has nothing to do with Win7. Something is faulty in the PS or motherboard. Whether that is a bad cap or diode or an almost broken pcb connection is irrelevant .